1. Field of the Invention
This invention relates to a color printing technique employing a plurality of kinds of ink.
2. Description of the Related Art
In recent years, color ink jet printers have come to enjoy widespread use as image output devices. A typical color ink jet printer employs, in addition to black (K) ink, several kinds of ink of the hues cyan C, magenta M, and yellow Y. Any color in a color image can be reproduced using these several kinds of ink.
In a printer of this kind, ink amounts of each useable ink are determined according to any color to reproduce a color image. Herein, the process of determining ink amount of each ink used in printing for color reproduction in this manner will be termed a “color separation process” or “ink color separation process.” Relationships of correspondence among color data of a color image and amounts of each color ink are pre-stored in a color conversion lookup table (LUT); during printing, amounts of each color ink at each pixel position are determined with reference to the LUT (see JP10-191089A, for example).
In preferred practice, the color conversion lookup table will be created such that colors represented by color data for a color image, and hue in particular, is reproduced faithfully. Thus, relationships of correspondence among color data and ink amount combinations are determined on the basis of reproduction results obtained through actual reproduction of a plurality of color patches or color samples. At this time, in order to achieve good color conversion accuracy of any color, a plurality of color patches are prepared so as to include a plurality of colors that differ in at least hue, saturation, or lightness. Human perception is particularly sensitive to deviation from achromatic color. Accordingly, it is desirable to faithfully reproduce color patches close to achromatic color, to improve color conversion accuracy for colors close to achromatic color.
In some instances, chromatic primary color inks actually used for printing have different coloring, depending on the ink. Coloring refers to the intensity of effect on reproduced color; for example, if ink quantity is small when an achromatic color is reproduced by a color mixture, it can be the that coloring is high. Here, with regard to color patches using inks with relatively strong coloring, in some instances the color balance will be biased towards the color of the ink with strong coloring. Significant bias, particularly for color patches close to an achromatic color, can result in lower accuracy of color conversion for colors close to the achromatic color.